Oxidic nanocrystals, in particular doped oxidic nanocrystals, such as for example europium-activated yttrium oxide (Y2O3:Eu) have been paid increasing attention in recent years for use as phosphors for illumination equipment (U.S. Pat. No. 5,455,489). For this reason, a very wide variety of oxidic nanocrystals have been tested for suitability for use as a phosphor. Consequently, corresponding processes and apparatus for producing nanocrystals of this type have also been proposed.
For example, to produce Y2O3:Eu, gas condensation techniques are known, cf. for example D. K. Williams et al., J.Phys.Chem.B 102 (1998), p.916. However, a drawback of these known processes is that the resultant oxidic nanocrystals are of highly amorphous structure and, in addition, have a monoclinic crystal structure. However, for the oxidic nanocrystals to be used as phosphors it is desirable for them to be in the cubic equilibrium phase, since this is when the light efficiency is highest.
To achieve this, hitherto the amorphous or monoclinic phase has been transformed into the cubic phase by an additional heat treatment at approximately 900° C. However, this entails considerable grain growth, and consequently the cubic phase is only present as a single phase at ultimate particle sizes of over about 50 nm. This in turn, on the one hand, means a high outlay in terms of energy and time for the production of nanocrystals of this type. On the other hand, the relatively large ultimate particle size considerably limits the possible applications of these oxidic nanocrystals which are produced using processes according to the prior art. Hitherto, it has not been possible to produce phosphors with small ultimate particle sizes, in particular of up to 20 nm, which have a high level of efficiency. The term ultimate particle size is understood to mean the grain size of the phosphor following-a heat treatment which may lead to partial agglomeration. The agglomeration can be classified as low if there is as yet no sintering of the grains at the boundary surfaces, although grains may already be arranged next to one another in rows. By contrast, the term initial particle size is used if the original grain size prior to a heat treatment is what is meant.